Google is terrified of users having access users control to their video content.
https://www.vice.com/en/article/youtube-tells-open-source-pr...
The only reason why we even know YouTube "has DRM" is because third parties have been able to plausibly allege DMCA 1201 circumvention claims against yt-dlp regarding a nebulously named "rolling cipher". These are not actual court findings of fact, just that you can say this in a legal filing and not immediately get your case thrown out on summary judgment. Which is a really low bar. Whether or not the rolling cipher actually qualifies as DRM is still an open question.
The way DMCA 1201 is written, basically anything intended to function as copy protection is considered DRM under the law. Like, those really annoying no-right-click scripts people used to put on sites probably could be argued to be DRM under DMCA 1201. However, in this case, there's a disconnect between the people offering the DRM (who don't actually claim it's DRM) and the people using it as DRM.
The most prominent public declaration of this stance occurred during the legal battle over youtube-dl (basically the ancesor of yt-dlp). While the RIAA initiated the initial 2020 DMCA takedown, Google's own technical implementation of the "rolling cipher" was the core of the argument.